ADA4932-2YCPZ-R2 datasheet(23/27 Pages) AD

23 / 27 page

Data Sheet

ADA4932-1/ADA4932-2

Rev. E | Page 23 of 27

The feedback resistor value is modified as a final gain

adjustment to obtain the desired output voltage.

To make the output voltage VOUT = 1 V p-p, calculate RF by

using the following formula:



























509

524

Desired

OUT

The closest standard 1% value to 509 Ω is 511 Ω, which

gives a differential output voltage of 1.00 V p-p.

The final circuit is shown in Figure 62.

ADA4932-1/

ADA4932-2

VOUT, dm

1.00V p-p

+VS

–VS

50Ω

499Ω

511Ω

VOCM

2V p-p

1V p-p

53.6Ω

RTS

25.5Ω

Figure 62. Terminated Single-Ended-to-Differential System with G = 2

INPUT COMMON-MODE VOLTAGE RANGE

The ADA4932-1/ADA4932-2 input common-mode range is

shifted down by approximately one VBE, in contrast to other

ADC drivers with centered input ranges such as the ADA4939-1/

ADA4939-2. The downward-shifted input common-mode

range is especially suited to dc-coupled, single-ended-to-

differential, and single-supply applications.

For ±5 V operation, the input common-mode range at the

summing nodes of the amplifier is specified as −4.8 V to +3.2 V,

and is specified as +0.2 V to +3.2 V with a +5 V supply. To

avoid nonlinearities, the voltage swing at the +IN and −IN

terminals must be confined to these ranges.

INPUT AND OUTPUT CAPACITIVE AC COUPLING

While the ADA4932-1/ADA4932-2 is best suited to dc-coupled

applications, it is nonetheless possible to use it in ac-coupled

circuits. Input ac coupling capacitors can be inserted between

the source and RG. This ac coupling blocks the flow of the dc

common-mode feedback current and causes the ADA4932-1/

ADA4932-2 dc input common-mode voltage to equal the dc

output common-mode voltage. These ac coupling capacitors must

be placed in both loops to keep the feedback factors matched.

Output ac coupling capacitors can be placed in series between

each output and its respective load.

SETTING THE OUTPUT COMMON-MODE VOLTAGE

The VOCM/VOCMx pin of the ADA4932-1/ADA4932-2 is internally

biased with a voltage divider comprised of two 50 kΩ resistors

across the supplies, with a tap at a voltage approximately equal

to the midsupply point, [(+VS) + (−VS)]/2. Because of this

internal divider, the VOCM/VOCMx pin sources and sinks current,

depending on the externally applied voltage and its associated

source resistance. Relying on the internal bias results in an

output common-mode voltage that is within about 100 mV of

the expected value.

In cases where more accurate control of the output common-

mode level is required, it is recommended that an external

source or resistor divider be used with source resistance less

than 100 Ω. If an external voltage divider consisting of equal

resistor values is used to set VOCM to midsupply with greater

accuracy than produced internally, higher values can be used

because the external resistors are placed in parallel with the

internal resistors. The output common-mode offset listed in the

Specifications section assumes that the VOCM input is driven by a

low impedance voltage source.

It is also possible to connect the VOCM input to a common-mode

level (CML) output of an ADC; however, care must be taken to

ensure that the output has sufficient drive capability. The input

impedance of the VOCM/VOCMx pin is approximately 25 kΩ. If

multiple ADA4932-1/ADA4932-2 devices share one ADC

reference output, a buffer may be necessary to drive the parallel

inputs.

HIGH PERFORMANCE PRECISION ADC DRIVER

Using a differential amplifier to drive an ADC successfully is

linked to balancing each side of the differential amplifier

correctly. Figure 64 shows the schematic for the ADA4932-1,

AD7626, and associated circuitry. In the test circuit used, a

2.4 MHz band-pass filter follows the signal source. The band-

pass filter eliminates harmonics of the 2.4 MHz signal and

ensures that only the frequency of interest is passed and

processed by the ADA4932-1 and AD7626.

The ADA4932-1 is particularly useful when driving higher fre-

quency inputs to the AD7626, a 10 MSPS ADC with a switched

capacitor input. The resistor (R8, R9) and capacitor (C5, C6)

circuit between the ADA4932-1 and AD7626 IN+ and IN− pins

acts as a low-pass filter to noise. The filter limits the input band-

width to the AD7626, but its main function is to optimize the

interface between the driving amplifier and the AD7626. The

series resistor isolates the driver amplifier from high frequency

switching spikes from the ADC switched capacitor front end.

The AD7626 data sheet shows values of 20 Ω and 56 pF. In

Figure 64, these values were empirically optimized to 33 Ω and

56 pF. The resistor-capacitor combination can be optimized slightly

for the circuit and input frequency being converted by simply

varying the R-C combination; however, keep in mind that

having the incorrect combination limits the THD and linearity

performance of the AD7626. In addition, increasing the bandwidth

as seen by the ADC introduces more noise.

ADA4932-2YCPZ-R2 数据表(PDF) 23 Page - Analog Devices

ADA4932-2YCPZ-R2 数据表(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

数据表下载